However, I don't believe that the structure you propose ("If I had worked tomorrow, I could not have agreed") is possible. Even in my most fevered imagination I cannot conceive of a situation in which it could be used.

"I cannot conceive of a situation in which it could be used." - How about someone decides to cancel a work contract and subsequently is offered an opportunity to go on holiday - surely they could then say to the inviter: "If I had worked tomorrow, I could not have agreed."?